According to the method of measuring the time t, the photoelectric distance meter can be divided into the pulse ranging method which directly measures the time and the phase ranging method which indirectly measures the time. The high-precision rangefinder generally adopts the phase type.
The ranging principle of the phase photoelectric rangefinder is: after the light emitted by the light source passes through the modulator, it becomes a modulated light whose light intensity changes with the high-frequency signal. The distance is calculated by measuring the phase difference φ of the modulated light traveling back and forth over the distance to be measured.
Phase method ranging is equivalent to using "light ruler" instead of steel ruler to measure distance, and λ/2 is the length of the light ruler.
In the phase distance meter, the phase meter can only measure the mantissa ΔN of the phase difference, but cannot measure the number of whole cycles N, so it cannot measure the distance greater than the optical ruler. In order to expand the measuring range, a longer optical ruler should be selected. In order to solve the contradiction between expanding the measuring range and ensuring the accuracy, two modulation frequencies, that is, two kinds of optical scales, are generally used in the short-range range finder. For example: long optical ruler (called thick ruler) f1=150kHz, λ1/2=1 000m, used to expand the measuring range, measure 100 meters, ten meters and meters; short optical ruler (called fine ruler) f2=15MHz, λ2/2=10m, used to ensure accuracy, to measure meters, decimeters, centimeters and millimeters.
Photoelectric rangefinder structure
1. Instrument structure
The host is installed on the upper part of the theodolite through the connector, and the theodolite can be an ordinary optical theodolite or an electronic theodolite. Using the optical axis adjustment screw, the launch-receiver optical axis of the host and the theodolite collimation axis can be located in the same vertical plane. In addition, the height from the horizontal axis of the rangefinder to the horizontal axis of the theodolite is the same as the height from the center of the target plate to the reflecting prism, so that the line of sight of the theodolite aiming at the center of the target plate and the line of sight of the rangefinder aiming at the center of the reflecting prism are kept parallel.
In conjunction with the reflective prism of the mainframe ranging, single prism (within 1500m) or triangular prism (within 2500m) can be selected according to the distance. The prism is placed on a tripod, and the centering and leveling are carried out according to the optical plummet and the long leveling tube.
2. Main technical indicators and functions of the instrument
The maximum measuring range of the short-range infrared photoelectric rangefinder is 2 500m, and the ranging accuracy can reach ±(3mm+2×10-6×D) (where D is the measured distance); the minimum reading is 1 mm; the instrument has an automatic The light intensity adjustment device can also adjust the light intensity manually when measuring in a complex environment; the temperature, air pressure and prism constant can be input to automatically correct the results; the vertical angle can be input to automatically calculate the horizontal distance and height difference; it can be preset by distance Carry out alignment staking; if you input the coordinates and elevation of the station, the coordinates and elevation of the observation point can be automatically calculated. Ranging methods include normal measurement and tracking measurement. The time required for normal measurement is 3s, and the average value of several measurements can be displayed; the time required for tracking measurement is 0.8s, and the ranging is automatically repeated at certain time intervals.
